Solution for http://blog.gainlo.co/index.php/2016/04/12/find-the-longest-substring-with-k-unique-characters/

Solver.java

package com.example;

import java.util.ArrayList;
import java.util.HashSet;
import java.util.List;
import java.util.Set;

/**
 * Solution for http://blog.gainlo.co/index.php/2016/04/12/find-the-longest-substring-with-k-unique-characters/
 * @author richardsantana
 */
public class Solver {

    public String getSubstring(String value, int uniqueChars) {
        Grouping grouping = new Grouping(uniqueChars);
        for (char character : value.toCharArray()) {
            String representation = new String().valueOf(character);
            grouping.addChar(representation);
        }
        return grouping.getGreatest().getString();
    }

    class Grouping {
        private List<Structure> structures;
        private Set<String> unique;
        private Structure greatest;
        private int maxSize;

        public Grouping(int maxSize) {
            this.structures = new ArrayList<>();
            this.unique = new HashSet<>();
            this.maxSize = maxSize;
            this.greatest = new Structure(this.maxSize);
        }

        public void addChar(String character) {
            if (!unique.contains(character)) {
                structures.add(new Structure(this.maxSize));
                unique.add(character);
            }
            structures.stream().forEach(structure -> {
                structure.addChar(character);
                if (structure.getSize() > greatest.getSize()) {
                    this.greatest = structure;
                }
            });
        }

        public Structure getGreatest() {
            return this.greatest;
        }
    }

    class Structure {
        private Set<String> unique;
        private int total;
        private StringBuilder stringBuilder;
        private int maxSize;

        public Structure(int maxSize) {
            this.unique = new HashSet<>();
            stringBuilder = new StringBuilder();
            this.maxSize = maxSize;
        }

        public String getString() {
            return this.stringBuilder.toString();
        }

        public void addChar(String string) {
            if (!containsChar(string)) {
                this.unique.add(string);
                total++;
            }
            if (total <= maxSize) {
                stringBuilder.append(string);
            }
        }

        public int getSize() {
            return stringBuilder.length();
        }

        public boolean containsChar(String string) {
            return this.unique.contains(string);
        }
    }
}